Norman Sperling's blog

Astronomical effects influence a lot of fields. But specialists in those studies don't always know enough astronomy to recognize what's really happening. Here's an example on a famous topic that no one would expect to have an astronomical dimension.

The highly-publicized hunt for "Nessie", the Loch Ness Monster, interests scientists and skeptics as well as the "crypto-zoologists" who hope that, in addition to the millions of small species that (naturalists assure us) remain to be cataloged, there may also be some unusually big ones. Discovering big new animals wouldn't violate anything scientific, and it would definitely be cool.

Nessie's setting is well known. In Scotland there lies a long, narrow, deep lake, Loch Ness, famous for its opaque waters. Sporadic reports from locals and tourists suggest that a large aquatic animal lives there, only rarely surfacing. A few ambiguous photographs and a lot of folklore support Nessie. The local hotels hope the hype continues to draw even more tourists than the pleasant landscapes and local culture earn on their own. Similar phenomena include "Champ" in Lake Champlain, Vermont, and "Ogopogo" in Okanagan Lake, British Columbia.

Just what the creatures might be, if real, remains to be demonstrated. I often heard plesiosaurs suggested, though these large marine reptiles are thought to have met extinction at the same time as dinosaurs, the end of the Cretaceous period, 65 million years ago. No plesiosaur fossils have been found in any later rocks.

"Remember the coelacanth!", the advocates remind us. These large primitive fish were also thought to be extinct, and now we have specimens of 2 species caught live - one species near the Comoro Islands and South Africa in the Indian Ocean, and the other in Indonesia. But the main reason to suspect a plesiosaur was its similarity to the "surgeon's photo", now admitted to have been a 1930s hoax.

A number of expeditions have sought Nessie, using more or less technological devices, and techniques of varying sophistication and likelihood of success. The one that produced the strangest result - often cited as the best scientific evidence for Nessie - was conducted in the summer of 1972. A sonar transducer (which converts sounds into electrical signals) was submerged 35 feet in the dark waters, connected by a long wire to analytical equipment aboard a boat. The transducer's signal traveled along that wire to amplifying electronics aboard the ship. If Nessie swam by the sonar detector, it would say so, even if Nessie stayed out of sight of the nearby submerged cameras. That is objective and neutral: no large signals means no large object, no Nessie; large signals can mean Nessie is there.

An hour after midnight on August 9, 1972, the sonar produced the peculiar strip-chart recording which is most often cited as showing the Loch Ness Monster. Though published1, this strip-chart is so different from conventional sonar output that even pro-Nessie studies quote the opinions of authorities, and several of those hedge2. Items by Rikki Razdan and Alan Kielar in the Skeptical Inquirer have disputed the positioning of the transducer (free-swinging or stationary), the stimulus for looking there and then (a dowser's signal), and the interpretation of the strip-chart. The matter remains controversial.

Despite the decades since then, I remember vividly where I was and what I was doing that week. I was in Springfield, Vermont, at the most famous astronomical convention in America. "Stellafane" is intended for people who make telescopes, but every year thousands who don't grind their own flock there too. I was attending my first Stellafane that very weekend. The sky was clear and dark. The Milky Way shone prominently. But everybody's attention was on something else. Brilliant green aurora - "Northern Lights" - flitted all around the sky. This was the finest display I have ever seen - the longest, the brightest, the most detailed and the fastest flickering, covering the most sky, right down to the south horizon.

In fact, this was one of the strongest auroras in decades, occasioned by one of the strongest solar flare outbursts recorded to that time. The Sun had just spat out a lot of charged particles, and they whipped Earth's magnetic field around, causing quite a lot of havoc. The storm induced electric currents in long wires, with many reports of damaging voltage and amperage variations. There were surges in the Canadian electric power grid; a big transformer exploded; short-wave radio communications were gravely disrupted; and sensitive electronic equipment was subjected to surges and flutters and spikes of current. Sky & Telescope magazine covered the event with no less than 5 articles, and J. A. McKinnon compiled a whole monograph on the event.

Much of Europe reported aurora and other electromagnetic phenomena from this solar storm. Loch Ness lies closer to the zone of greatest auroral intensity, the "auroral oval", than most of Europe.

The peculiar sonar reading occurred at just the time of the second-greatest peak of magnetic intensity. But the Loch Ness investigators didn't report the aurora. Most likely it was cloudy there, as it is about 90% of the time. Even had it been clear, their attention would have been focused down toward the waters, and it would be entirely understandable if they didn't notice diffuse phenomena occurring behind them and apparently unrelated to their interests. They did, however, note that "the hair went up on the backs of their necks" - an effect well-known in electrical demonstrations - though they interpreted that as "primitive instincts" that "there was something ominous in the loch that night"3.

One sensitive electronic instrument, using a long wire, did give a peculiar reading just when an exceptionally strong gust of solar wind swept by Earth, just when hair rose on their necks. The least-strange interpretation is that this sonar recorded the magnetic storm, rather than the Loch Ness Monster. This might explain why the reading from the Loch Ness equipment is so strange that it requires expert interpretations, and why those say different things.

If so, the Loch Ness investigators may deserve a more charitable treatment than some skeptics have given them. They reported what their instrument told them, and that instrument gave a reading that is possible to interpret as data confirming an unusually large object or creature. The hair-raising clue alone was too little to pick up on. The aurora was probably hidden by clouds, and even if visible would not likely attract their attention, let alone their suspicion. And while atmospheric scientists and astronomers would connect the aurora to the strangeness of signals riding long wires, few other scientists would suspect their instruments of telling them anything beside what they're designed to tell.

Absence of evidence is not evidence of absence, so you can still root for Nessie. But the scientific evidence (with the sonar reading resulting from aurora, and the "surgeon's photo" an admitted hoax) is very meager.

Everything people deal with is embedded in a cosmic setting. The better people understand the cosmos, the better they can deal with it.

Astronomical news travels fast. Within hours, first details of a comet or nova get to astronomers all around the world. Of course, all sorts of news travels fast these days, which is often cited as a wonder of modern technology. It may be a surprise to learn, therefore, that long before the global media culture arose, before television, and even before radio, astronomers set up their own system to spread scientific alerts. Astronomers recognized that national hostilities inhibited their research, and so, long before the United Nations or the League of Nations, astronomers designed procedures to evade such hostilities.

For more than a century, when the internationalist needs of astronomical communications have collided with nationalist politics, astronomy has consistently won far out of proportion to its political strength. Astronomy's successful internationalism is epitomized by the Central Bureau of Astronomical Telegrams, founded in 1882. When the International Astronomical Union was founded in 1919, its Commission 6 assumed responsibility for the Bureau. The Bureau has overcome every administrative, economic, and military obstruction the world has thrown at it. It even kept functioning without interruption through World Wars I and II, disseminating news of transient phenomena to all subscribers regardless of nationality.

How Cosmic Events Impose Internationalism on Astronomers

Several important categories of celestial phenomena occur unpredictably in time and in sky location. Comet and asteroid discoveries, nova outbursts, supernova explosions, and unexpected behavior or previously-known objects (such as sudden storms on Mars or Jupiter) are all phenomena which astronomers feel urgency in studying. Astronomers must observe them quickly, with virtually any available instrument.

"Among such discoveries are those of planets and comets, or of bodies which are generally so faint as not to be seen, except through the telescope, and which being in motion, their place in the heavens must be made known to the distant observer, before they so far change their position as not to be readily found. For this purpose the ordinary mail conveyance, requiring at least ten days, is too slow, since in that time the body will have so far changed its position as not to be found, except with great difficulty; and this change will become the greater if the body is a very faint one, for in that case it could only be discovered on a night free from moonlight, which of necessity in ten or twelve days must be followed by nights on which the sky is illuminated by the Moon, and all attempts to discover the object would have to be postponed until the recurrence of a dark night. Indeed, even then the search often proves in vain; and it is not, in some cases, until after a set of approximate elements are calculated and transmitted, that the astronomers on the two sides of the Atlantic are able fully to cooperate with each other." (Joseph Henry, quoted in Monthly Notices of the Royal Astronomical Society, 1874, p. 185)

The explanation continues:

"Although the discovery of [minor] planets and comets will probably be the principal subject of the cable telegrams, yet it is not intended to restrict the transmission of intelligence solely to that class of observation. Any remarkable solar phenomenon presenting itself suddenly in Europe, observations of which may be practicable in America several hours after the Sun has set to the European observer; the sudden outburst of some variable star similar to that which appeared in Corona Borealis in 1866; unexpected showers of shooting-stars, &c., would be proper subjects." (Monthly Notices of the Royal Astronomical Society, 1874, pp. 185-186)

Comets and Asteroids

Comets can be found in any part of the sky, at any brightness, and moving at any rate in any direction. When a comet is discovered, the discoverer immediately tells the Central Bureau for Astronomical Telegrams its position, brightness, and (if determined) its apparent direction and rate of motion. Comets are frequently found under conditions in which the discoverer cannot track them long enough to establish their rate and direction of motion against the background of fixed stars - they may be near the horizon and setting, they may be found just before dawn glares them out, they may be spotted in a sky that quickly clouded over, or personal circumstances may preclude further observing. Comets must be recovered before they move too far from the discovery position, so that their orbits can be determined, and from those orbits, the ephemerides for future sightings. Given two days from first sighting, a comet can move several degrees away from the first position, and fade by two magnitudes. The differences in position and brightness grow so great that a discovery unobserved for three or four days is in grave danger of becoming utterly lost. It is extremely important that astronomers elsewhere be alerted so they can find the unexpected interloper and pin down its positional and visual characteristics. So the Central Bureau immediately cables professional observatories and reliable amateur observers west of the first reporting station, in longitudes where it is still nighttime, hoping that at least one will at that moment have clear skies and staff and equipment they can shunt to hunt the comet. As the hours tick by, observatories farther and farther west around the Earth join the hunt, until sufficient observations are confirmed to make the comet unlikely to be lost. Usually, by the time the comet is next observable from the discovery location, confirming observations are in hand, and the first reporting observer learns whose names will be attached to that interplanetary iceberg. It is invariable in the discoverer's interest to alert the whole world absolutely as soon as possible.

There are also minor planets (asteroids) swarming around the inner solar system by the thousands. A few of these orbit strikingly close to Earth. At such approaches, they brighten dramatically, making a rock barely a kilometer across visible to amateur telescopes. Simultaneously, they sprint across the sky, making it hard to take a time-exposure (they move too fast) and hard to make an accurate positional fix (requiring a trained team with cross-hairs and setting circles), just when it is most important. Such asteroids can actually collide with our planet, with results presently popularized regarding the Cretaceous/Tertiary extinction.

Stellar Outbursts

Eruptive variable stars ― novae and supernovae ― are very important, fast-changing stages late in stellar evolution. Unfortunately, they occur so unpredictably that observations of the brief, insufficiently-understood brightening phase(s?) are very rare. Therefore, it is important to obtain both brightness measurements and spectra of all such outbursts absolutely as early as feasible.

Novae and supernovae do not move against the background of stars, and so a single positional fix suffices to locate them. Their brightness and spectral behavior, however, change radically, and as rapidly as anyone has yet measured ― brightness flickerings of tenths-of-seconds are known for at least a few novae. Therefore, brightness requires continual monitoring. The spectrum also changes enormously as assorted layers of the erupting star lunge through wildly different temperatures, pressures, and densities. This gives some of our best information about the insides of stars, and so it is again critically important to obtain spectra very frequently, especially when the eruption is fresh.

Again, observatories around the globe must be alerted because longitude and weather patterns will only allow observations at some of them, and other circumstances of equipment and personnel further reduce the number at which the critical observations can be made. (For example, sensing hardware at the tail-end of the telescope, such as a photometer or a spectrograph, is fitted to a major telescope in a many-hours-long daylight operation, and usually remains attached for an observing run lasting several consecutive nights. If the telescopes at a well-located, well-equipped, well-staffed, and cooperative observatory are outfitted for other research and incapable of the type of measurements needed for the fresh nova, that observatory can make no contribution, however eager it might be to help.) So again, the alert must go out as rapidly and broadly as technology and funding permit.

All astronomers know these factors. They are inherent in the astronomical objects being studied and utterly independent of astronomers of planet Earth. It is up to astronomers to accommodate their research methods to their objects of study.

Premature concepts in International Information Flow

Communications technology, not politics, has been the limiting factor in dissemination of astronomical news. Though it has always been obvious to astronomers that instant bulletins would be of great value, they have also appreciated the impossibility of achieving that till quite recently. In Tycho's era, centuries before the Universal Postal Union regularized international mail, a central location would have served best.

"Tycho Brahe ... keenly felt the importance of closer intercourse between fellow astronomers .... he tells that before King Frederik II of Denmark offered him the island Hven as site for an observatory he had had plans for settling in Basel, one of the reasons for this choice being that Basel is located so to speak at the point where the three biggest countries in Europe, Italy, France, and Germany meet, so that it would be possible by correspondence to form friendships with distinguished and learned men in different places. In this way it would be possible to make my inventions more widely known so that they might become more generally useful.' Into these remarks may be read a desire for a centre, from which useful astronomical news could be distributed. Such a centre, however, took its time to materialize." (Vinter Hansen, 1955, p. 17)

In the early 1800s, printing and postal improvements began to make a newsletter practical, and Astronomische Nachrichten, the first international astronomical newsletter, was born. Its founder, Heinrich C. Schumacher, an expert celestial mechanic, established it in 1823 at his observatory in Altona, Holstein, then the second-largest city in Denmark (Beekman, 1983, pp. 488-489). Altona was immediately northwest of Hamburg, Hanover, across the Danish boundary set in 1815 at the Congress of Vienna. (Altona is now well within Hamburg city limits.) During the Prusso-Danish War of 1848,

"the existence of his observatory at Altona was in doubt. The astronomers throughout the world rallied for the protection of Schumacher, and almost every civilised country made through its representatives urgent appeals to the Danish Court that the observatory and its respected director should be spared and protected. Lord Palmerston, pressed to action by the Council of [the Royal Astronomical] Society, obtained assurances from the Danish Government that neither the Professor nor his establishment should be affected." (Stratton, 1934, p. 363)

Schleswig and Holstein were incorporated into what became Germany, and upon Schumacher's death in 1850, Astronomische Nachrichten was transferred to C. N. Adalbert Kruger, celestial mechanic at the Kiel Observatory, on the north edge of Holstein.

Telegraphy was improving markedly during this period. The first astronomical telegram center was established by the Imperial Academy of Sciences in Vienna in 1869, limited to comet discoveries but with a gold medal for incentive. (Gingerich, 1968, p. 37; Vinter Hansen, 1955, p. 18)

The Atlantic Cable

The need for a broader service was apparent to "some of the principal astronomers in Europe and America," however, and so C. H. F. Peters, the prominent director of Hamilton College Observatory in Clinton, New York, laid the need before the Committee of the Smithsonian Institution. They "immediately applied to the Directors of the Associated [Trans-Atlantic Cable] Companies, who at once granted the free use of all their lines for the object in question, both from Europe and America, for a limited number of telegrams during each year." (Monthly Notices of the Royal Astronomical Society, 1874, p. 185)

"A very important concession has been made to the Smithsonian Institution by the Directors of the Associated Trans-Atlantic Cable Companies, who have agreed to transmit gratuitously between Europe and the United States, a limited number of short messages on astronomical subjects. Under this arrangements two telegrams have already been received from the United States by the Astronomer Royal, who on his part has undertaken, at the request of Dr. Henry, Secretary of the Smithsonian Institution, to forward from Europe any message announcing an important astronomical discovery. The Directors of the Associated Companies have consented that ten messages, of ten words each, may be sent free over the cables annually. This liberal concession on the part of the Directors cannot be too highly appreciated by astronomers generally, and especially by the fellows of this Society.

"In conformity with this agreement the Astronomer Royal will be prepared to forward any important astronomical message, limited to ten words, which may be sent to him for this purpose from the principal European astronomers. Royal Observatory, Greenwich, April 8, 1873." (Airy, 1873)

The matter was further elaborated on 10 months later:

"The great value of this concession on the part of the Atlantic Telegraph and other companies cannot be too highly prized, and our science must certainly be the gainer by this disinterested act of liberality. Already [minor] planets discovered in America have been observed in Europe on the evening following the receipt of the telegram, or within two or three days of their discovery. The Council are glad to be able to announce that the Director of the Imperial Russian Telegraph has also given permission for the free transmission of messages relating to new astronomical discoveries within the boundaries of the Russian Empire." (Monthly Notices of the Royal Astronomical Society, 1874, p. 186)

The Central Bureau for Astronomical Telegrams

While this was a major advance, things were not quite perfect. There were technical difficulties with transmission errors, particularly in numbers for coordinates (causing observers to look in the wrong place), and "national vanities had showed up." (Vinter Hansen, 1955, p. 18, referring to Wilhelm Forster, 1879, p. 345)

So Forster proposed establishing a central news bureau, whose creation was spurred by general agreement that bulletins on the great September comet of 1882 were "thoroughly inadequate." (Vinter Hansen, 1955, pp. 18-19; Forster 1881, Forster 1882, and Kruger 1883) Forster nominated a committee of eight to set up the Centralstelle in 1882 in Kiel with Kruger as director. (Stratton, 1934, p. 363) The director would decide which items required expensive telegrams and which could be relegated to the established ― and far more economical ― columns of Astronomische Nachrichten. By this time the journal was no longer the personal property of its editor; in 1881 a share of control was assumed by the Astronomische Gesellschaft, which had been founded at Heidelberg in 1863.

"Though primarily German, this body has always been alive to the value of international co-operation in astronomy. Its statutes specifically stated that its membership was not bounded by nationality, and the list of those attending its first meeting includes members present from ten countries." (Stratton, 1934, p. 363)

From the 1880s to the beginning of World War I, the Centralstelle operated smoothly at Kiel, under Kruger and his successor, H. Kobold.

But by no means was it astronomy's only international cooperative venture. In 1887, Observatoire de Paris hosted the International Astrophotographic Congress at which the monumental Carte du Ciel project was begun (Winterhalter, 1889). The project required meetings every few years to monitor progress, and gradually these meetings (usually in Paris) came to include extra sessions on astronomical topics unrelated to the Carte du Ciel itself. Other international astronomical meetings were also held, for example at the opening of Yerkes Observatory in Williams Bay, Wisconsin, in 1898, and at the St. Louis World Fair in 1904.

The guns of August, 1914, however, severed telegraphic and postal links between the belligerents. It became impossible to communicate directly between, say, the Royal Greenwich or Paris observatories and Kiel.

Prof. Kobold therefore ceded the Central Bureau for Astronomical Telegrams on 3 November 1914 to another celestial mechanic, Prof. Elis Stromgren, director of Copenhagen Observatory, in adjacent but neutral Denmark, for the duration of the hostilities. Kobold and Stromgren knew each other well ― Stromgren had earlier been an assistant at Kiel and knew how the Central Bureau worked. (Vinter Hansen, 1955, pp. 19-20) In the early months of World War I most people thought the war would be brief, but it dragged on almost until the end of 1918.

"It was possible for [Stromgren] all through the war to keep up astronomical intercourse and satisfactory news service between astronomers all over the world." (Vinter Hansen, 1955, p. 20)

Losing the Peace

The problems with national antipathies were thus overcome during World War I. However, it proved very difficult to keep a lid on after the Armistice was signed. During the war, astronomers could travel little and many were occupied with war-related problems. When peace finally came, astronomers could once more travel, but residual jingoism and bitterness prevented many in the Allied countries from wanting anything to do with anyone in the Central Powers. Thus, in 1919 when the International Astronomical Union was formed as part of the International Research Council, only entente powers were invited to adhere at first, followed shortly by neutral countries.

"Now each of the pre-war organizations for international cooperation was concerned with its own particular branch of astronomy; for they had come into being one at a time, as the necessity for them had been felt. It was clear, however, that it would be much more convenient for every one concerned if they could be brought together into a single body. And so, when these various organizations died a natural [sic] death as a result of the war, it was decided to revive them, not in their original form, but as branches of an all-embracing International Astronomical Union. This was accomplished at a meeting in Brussels in 1919 of the leading scientists of the allied countries; and in 1922 the first meeting of the International Astronomical Union was held in Rome .... At first the I.A.U. consisted of thirty-two separate 'commissions,' fourteen of which could be regarded as the direct descendents of pre-war organizations." (Waterfield, 1938)

Losing countries were not welcome to adhere for many years ― though "Astronomers from countries not adhering can attend meetings as visitors and take part in the discussions without voting." (Waterfield, 1938)

The national antipathies resulted in some astronomers refusing to deal with Copenhagen, feeling it too closely connected with Kiel, which was German and therefore tarred with atrocities. The popular director of Observatoire de Paris, Benjamin Baillaud, served briefly as intermediary between such people and Copenhagen. Then the brand-new International Astronomical Union, under provisional leadership, set up its own Central Bureau for Astronomical Telegrams ― neither at Kiel nor Copenhagen, but seeking a more neutral location (Gingerich, 1968, p. 38), at Uccle, the Belgian royal observatory in Brussels, under Prof. G. Lecointe, starting 1 January 1920. Copenhagen served as intermediary between the new Uccle and the old Kiel bureaus until the IAU held its first full meeting, in 1922. The IAU thereupon transferred its Central Bureau from Uccle back to Copenhagen as of October, 1922. From then until his death, Elis Stromgren directed the IAU Central Bureau for Astronomical Telegrams from Copenhagen Observatory and maintained friendly relations with Kiel. (Hoffleit, 1947, p.6)

World War II

Denmark was unable to keep out of World War II. Germany invaded in April, 1940, and occupied Denmark for the rest of the war. Elis Stromgren succeeded in keeping the Bureau communicating world-wide, however. "He even obtained permit to send code telegrams, via the Lund Observatory, [in neutral] Sweden, to subscribers in allied countries, and the IAU circulars, although often late in arriving, generally did reach their wide-spread destinations." (Vinter Hansen, 1955, p. 20) The routes were often circuitous: IAU Circular 901, for example, arrived at Harvard Observatory in 1942 bearing an Brazilian stamp (Overbye, 1980, p. 93); it doesn't say how the data got from Sweden to Brazil. In the few times when Stromgren could not even get messages out through Sweden, he made connections through Zurich, Switzerland, also a neutral for relaying telegrams worldwide. Connections could always be made either through Sweden or Switzerland. (IAU Transactions, Vol. VII, p. 86)

There was, however, a distinct decline in traffic in 1943-44, at the worst period of the German occupation of Denmark. (IAU Transactions, Vol. VII, p. 87) The decline probably reflects the diversion of skywatchers into the war effort, thereby reducing the number of sky events noticed, as well as the difficulty of telling Copenhagen about them.

After World War II, communications links became less troublesome, and more astronomers again looked skyward and reported discoveries. The effort the Central Bureau once spent routing messages was now devoted to processing the increased data.

Elis Stromgren died in 1947 and was succeeded by his assistant, Julie Vinter Hansen. She had assisted at Copenhagen for many years, though she spent World War II at Lick Observatory in California. She returned to Denmark when the war ended and resumed her assistantship under Stromgren, succeeding to his work upon his death. (Ashbrook, 1960, p. 328) She remained director until her own death, in 1960. By 1964, the ongoing duties of the Bureau were overwhelming her assistant and successor, K. A. Thernoe, and so Fred Whipple offered to transfer the Bureau to the Smithsonian Astrophysical Observatory, which cohabits with Harvard Observatory in Cambridge, Massachusetts; Whipple was director of both Smithsonian and Harvard observatories. Owen Gingerich directed the Bureau when it moved to Massachusetts.

The Central Bureau at the Hub of the Universe

"Thanks to B. G. Marsden, Director of the Bureau, and to the continued support of the Smithsonian Astrophysical Observatory, astronomers have at their disposal an efficient and relatively inexpensive mechanism for prompt cooperation that must be the envy of scientists working in other fields." )P. Simon, President of Commission 6, in IAU Transactions, Vol. XVI A1, 1976, p. 195)

As early as 1883, Harvard College Observatory served as North American telegram relay center ― receiving all telegrams from the Centralstelle and relaying them to American subscribers. Several similar branch offices around the world have sprung up and died off sporadically over the last century, but Harvard has the longest and steadiest record. Beginning in 1926, the "Harvard Announcement Cards" constituted this continent's equivalent of the IAU Circulars. (Gingerich, 1968, pp. 37-38) During Harlow Shapley's reign as Observatory Director, they were all signed "Harlow Shapley," but while he oversaw most of the operation, many of the orbital elements and ephemerides they contained were actually composed by others ― by Leland Cunningham during his assistantship there, 1935-42. The Shapley signature proved very useful toward acquiring donations; Shapley was able to point to important work done despite a pathetically small endowment. (Cunningham, 1985)

The bulk of the labor consisted of the tedium of decoding Copenhagen telegrams, re-coding them for American distribution since until World War II Copenhagen and Harvard used different telegraphic codes, and conveying them to the telegraph office. Initially the Harvard cards were simple relays of Copenhagen data, but they grew to include other ephemera of American interest, as well as data on their way to Copenhagen. (Cunningham, 1985) When the Bureau was transferred, the Harvard Announcement cards terminated with card 1676, and were succeeded by IAU Circular 1884; Circular numbers reached 5152 by the end of 1990.

The main part of the operation is carried on today in the office of Brian Marsden with active VAX and IBM computer terminals as well as Telex and assorted other electronic links through the Smithsonian's telecommunications center until 1978, and managing its own since then. Subscribers with personal computers could access the very latest listings by modem[, and since the mid-1990s, on the World Wide Web]. Marsden has an assistant, Daniel Green, and immediate access to the entire Center for Astrophysics staff and library ― one of the largest concentrations of astronomical expertise on this planet. Routine celestial mechanics still makes up the bulk of the work, with special attention to keeping new-found objects from getting lost.

The main consideration is to pin down the ephemeral events in the heavens, but where earthly circumstances allow options, lesser criteria do enter. The judgement of a thinking, feeling, experienced human is vital when an incoming message has to be interpreted. How likely is the source to be mistaken, or trying to pull a fast one for quick glory? Which observatories should be dunned for confirmatory monitoring ― who are the assigned observers this evening, who sounded eager last time, who felt it a burden, who could use a little favorable notice, who should get no more than necessary?

Commission 6

Commission 6 is largely concerned with the administrivia of keeping the Central Bureau operating. It decides on formats, amendments to the standard transmission code, subscription billing intervals, and the like. Commission business meetings at Congresses are brief, cut and dried. Reports of the Director and President are distributed in advance, consensus always reigns on major issues, and discussion achieves consensus on minor ones. Membership has largely been intended to represent the message relay centers, wherever in the world they may be, plus a few comet and orbit experts. The vast bulk of the work is done not by the commission but by the Central Bureau. (Cunningham, 1985)

Even within the IAU, Commission 6 has striven for the broadest possible base:

"[Marsden] noted that all Commission members present at the Meeting were, without exception, also members of Commission 20 [Positions and Motions of Comets, Minor Planets, and Satellites], which might give the impression that Commission 6 was mainly a sub-section of Commission 20. This was by no means the case, and to stress this fact a special effort had been made some years previously to include representatives from other Commissions. However, these had taken little or no part in Commission 6 activities. The reason might be that rapid dissemination of astronomical data still remained, as in the past, mainly of interest to observers of minor planets, comets and related objects, while it was evident that workers in other fields were entirely satisfied with the operation of the Bureau." (IAU Transactions. Vol. XVIII B, 1982, p. 79)

Tokens of Appreciation

The Bureau's funding has varied markedly over the years. Typically the host institution underwrites either staff salaries or overhead, sometimes both. In Copenhagen, until October, 1929, an unnamed "outside source" provided financial assistance. (IAU Transactions, Vol. III, 1928, p. 276) Since 1929, the IAU has paid yearly subsidies toward the Bureau's work, but these have grown far slower than expenses. While in Copenhagen, very liberal donations were made by the Danish Rask-Orsted Foundations. Additional donations come from other Danish foundations. These were both contributions toward operating expenses and advances against subscription income uncollected due to World War II, when most recipients of telegrams were prevented from paying for the service. (IAU Transactions, Vol. VII, 1948, p. 276) Those loans were eventually paid back, though the last of them lingered years after the guns fell silent. The foundations' motives probably reflected local pride about being the world center of something.

Since the 1950s, subscriptions of various categories have covered the vast majority of the Bureau's operating funds. Originally these only paid for outgoing messages, but they have been increased deliberately since the 1965 move to Harvard to cover as much of the infrastructure (computing time, processing incoming messages, and part of the staff salaries and building overhead). From the time of the move, the Smithsonian Astrophysical Observatory paid for the director's salary, while 1965 and 1966 subscriptions totaled $7,328 – and the IAU subsidy was $666/year. This "pays for very little of the actual operation of the Bureau, but it provides considerable moral support and gives the operation a desirable flexibility that it would not otherwise have." (Gingerich in IAU Transactions, Vol. XII A, 1967, p. lxxxii) Certain low-priority types of message now also trigger a line-charge, analogous to the page-charges of professional journals, generating further income. IAU's cash contribution, though increased to 10,000 Swiss francs per triennium, is now less than 5 percent of the Bureau's actual running costs. It remains, however, "tangible proof of the high regard in which the Bureau is held by the Executive Committee, and a letter to this effect received from the General Secretary was noted with appreciation." (Commission President Jan Mers in IAU Transactions, Vol. XVIII B, 1982, p. 79) It is "used partly to ensure that Circulars are supplied to important observatories in regions of the world whence it is difficult or impossible for us to receive payment." (Marsden in IAU Transactions, Vol. XV A, 1973, p. 17)

Conclusions: Strident Internationalism Victorious

The Central Bureau for Astronomical Telegrams has achieved its goals for more than a century. It receives information from all around this planet about transient events all around the universe. It disseminates the data to all observers it can identify as needing such service. The Bureau's administration has changed only once, from an independent commission to an International Astronomical Union commission. At that same time, more than a dozen other international astronomical programs became IAU commissions; almost half of the new IAU's commissions traced their heritages to previous arrangements. The Bureau moved from Kiel to Copenhagen, briefly to Brussels, and eventually to Cambridge, Massachusetts, always to take advantage of the best site to and from which to relay messages, consistent with the availability of a first-class celestial mechanic. Citizens of many countries participate in Commission 6. The Bureau's avowed purpose has authorized methods of data transmission that evade obstructions imposed by wars and any other causes. It has succeeded in this at all times, employing relays at observatories wherever they may be handy. Thus the Central Bureau for Astronomical Telegrams demonstrates astronomy's long-term, ongoing, strident internationalism.

Winterhalter, Albert G. The International Astrophotographic Congress and a Visit to Certain European Observatories and Other Institutions: Report to the Superintendent. Appendix I to the Washington Observations for 1885. Government Printing Office, Washington, D.C., 1889, pp. 38, 42-46, 68-71.

Until the 1900s, virtually all humans knew the appearance of the dark night sky. Even unschooled urbanites knew some constellations and planets. By 1909, light pollution made authors admonish readers to do their skywatching from the countryside rather than the city. The warnings have escalated along with the light pollution. Light pollution's effect on professional and volunteer observational astronomy, along with telescopes' changing focal ratios, largely determine which kinds of astronomy are done in which institutions. In times and places where individuals perceive little possibility to change their culture, astronomers cope as best they can. When activism earns results in other cultural matters, astronomers sometimes become activists to fight light pollution. Despite winning some battles, the war against light pollution is still being lost, so a different approach is suggested.

First-World Light Pollution

Attention to light pollution depended, and still depends, upon local and cultural conditions. Geography, meteorology, energy consumption methods, economics, technology, politics, and demography all mold local circumstances, and generate objectionable levels of light pollution at different times in different places. Light pollution's interrelationships with popular astronomy, professional and amateur research, instrumentation, and observing sites demonstrate its strong influence.

Light pollution is high in the consciousness of those who suffer from it nightly, but writers used to dark skies rarely mention it. Writers in smoky cities bemoan smoke, and writers in electrically-lit cities bemoan electric lights – though writers in cloudy climates bemoan the clouds at least as loudly. So the writers talk about whatever interferes with their skywatching. And none ever hints that anything can be done to avoid it, except travel.

By 1866, and perhaps earlier, the first caveats about light pollution crept into the popular astronomical literature. Sir John Herschel (1792-1871) noted the problem (Crawford, personal communication). Amedee Guillemin (1866) wrote that the dimmest stars are effaced altogether "in the great centers of population, by the illumination of the houses and streets." The haze enveloping Paris and London, and the smoke filling the skies of many cities, was the primary obstruction, however. It was largely wood and coal smoke plus street dust; we would call it air pollution. It still wasn't too bad, because in 1869, Edwin Dunkin was still able to advocate urban skywatching: "It is of no consequence, therefore, in what part of London, or its neighborhood, the observer is located. It may be in the heart of the city …" But another Londoner, John A. W. Oliver, wrote of the Zodiacal Light in 1888, that "the less luminous portions cannot be well seen in a town where there is smoke illuminated by gaslight, or where the electric light is in use, as in the city of Boston, where Searle finds it no longer possible to observe the Zodiacal Light satisfactorily." Therefore, light pollution has definitely interfered with astronomical observing since the late 1800s.

From that time on, professional astronomers have almost always considered seeing conditions when locating new observatories. In addition to climate and altitude, they include light pollution as a prime consideration. While San Jose, Flagstaff, and Pasadena were still small, dim towns, Lick, Lowell, and Mount Wilson Observatories grew on nearby peaks – only to suffer terribly in the light of recent developments.

Light pollution became a pressing topic in British and American – and even Austrian – popular astronomy books and amateurs' observing manuals from 1909 on. The problem escalated, both in the skies and in print, as the 1920s yielded to the 1930s, with authors preferring more and stronger warnings.

Then World War II blacked out major cities. All of a sudden, generations of urbanites who had never seen the starfilled sky clamored for books about this splendid vision, and despite wartime paper rationing, England (among other places) generated volumes to explain the sky. These old-fashioned star-watching manuals addressed readers who were seeing the dark sky as a novel phenomenon.

After the War, "the lights came on all over the world." The skies lit up again, urbanites lost touch with the stars, and books returned to their warnings to seek dark – typically rural – skies. Now, great numbers of city children attending planetarium shows cannot relate to the dark sky shown because they have never experienced such a sight in nature. Decisions made by generations unfamiliar with nature often seem to ignore it, with results ranging from regrettable to catastrophic.

And, now that most towns are light-polluters, observatories have been pushed farther away – Fort Davis, Kitt Peak, Mauna Kea, Tenerife, Las Campanas. A peak's isolation is now one of its prime astronomical assets, almost regardless of the difficulties imposed on construction and operation. Astronomers working on low-surface-brightness problems depend on such facilities. Observational astronomers working in light-polluted areas are restricted to high-surface-brightness targets, most of which are far from the forefront.

Fast Optics Accent Light Pollution

An often-overlooked element in the rising clamor has been the change in focal ratio of both professional and amateur telescopes. Most 19th Century telescopes were f/15 to f/20 refractors. Such instruments excel for positional astronomy, as well as with high-surface-brightness objects like planets and double stars ... and are relatively unbothered by diffuse skyglow because they operate at high magnification with small fields of view. That is why most remain in the cities and campuses where they were first set up, and why so few have been moved to remote mountaintops.

Since World War II, however, the overwhelming majority of professional and amateur observing has been accomplished with reflectors. It is relatively easy to make reflectors optically fast (a difficult problem for refractors), and preferable for cutting exposure time, and lowering the cost of mounting and housing. Fast optics concentrate diffuse light, so they excel for deep-sky observing of low-surface-brightness nebulae, clusters, and galaxies ("Of Pupils and Brightness", Sperling 1985).

With their large, fast reflectors, amateurs have waxed enthusiastic for deep-sky observing. Several people have called this a result of the aperture explosion, which is perhaps spurred by economics as well as technology. But the faster focal ratios have been at least as much a factor as the wider apertures. And it is just those fast focal ratios that yield the wide fields of view and low magnifications that accent light pollution. Thus, when amateurs poured huge sums into huge telescopes to see huge distances, they found light pollution glaring back at them, preventing them from enjoying the view they had invested so much to see.

Political Activism

In the United States in the 1960s and '70s, light pollution increased precipitously with population, more brightly-lit cities, and suburban sprawl; amateur focal ratios sped up greatly, amateur apertures exploded enormously – and political activism spread from the Civil Rights movement to opposing the Viet Nam war, and to popular causes in general. This national mood gave American professional and amateur astronomers the idea to become activist – actually fighting light pollution, instead of merely running away from it.

The struggle in the United States, Britain, and Canada is largely political. Mostly through amateur astronomy clubs, American and British hobbyists have repeatedly challenged offensive lighting, and have won several notable battles (itemized in Sperling 1978, 1980, and 1986).

The first major salvo came from Tucson-area professional observatories in December 1971. Subtitled A Guide for Businessmen and the General Public, it described the problem and the ordinance they proposed to cope with it (Steward 1971). The ordinance passed, and Southern Arizona astronomers have been leaders in the struggle ever since. They continue issuing assorted publications (such as Crawford 1985), monitor Arizona's slowed-but-still-growing light pollution, and undertake such other strategies as organizing IAU Colloquium 112 and the International Dark-Sky Association.

In 1973 and 1974, the United States endured a painful fuel shortage of political origin. Professional and amateur astronomers seized upon an anti-waste strategy for fighting light pollution, and the struggle took on its current aspect. Kurt Riegel published his survey paper in Science (Riegel 1973), and Kitt Peak National Observatory issued another, more comprehensive book explaining the problem and recently-passed laws restricting the growth of outdoor lighting (Hoag and Peterson 1974).

In Manassas, Virginia, at the 18 May 1974 Middle East Regional convention of the Astronomical League, an unprecedented array of leading amateur astronomers passed and published a battle plan. It was authored by Jack Betz of Harrisburg, Pa., and championed by the usually-reactionary Bob Wright, as well as several much more activist leaders, including myself. Betz advocated a succession of measures, from shielding the observing area, through contacting neighbors and utilities, to seeking governmental action (Betz 1974). Several regions of the Astronomical League (the American federation of astronomy clubs) have sponsored activist projects fighting light pollution since then, most notably the South East and Great Lakes regions. These efforts typically collect and distribute anti-light-pollution campaign literature, for use by local astronomy clubs. Of course, every amateur works on his own time with his own resources, so these efforts flare up and subside sporadically.

On 1 November 1974, the Toronto Centre of the Royal Astronomical Society of Canada's "Sky Brightness Programme" issued a manual telling how to measure light pollution photometrically. This was another salvo in amateur activism, taking a scientific-measurement stance (Berry 1974). It has been pursued professionally by Arthur Upgren and others.

One of my articles (Sperling 1980) pointed out that astronomers most often succeed when they exercise personal connections with government officials – a very depressing conclusion for societies so proud of their democracy. This may result from a cultural climate in which the kinds of people who become public officials rarely know much science, and astronomy is so far from their awareness that they don't sufficiently understand an astronomer with an odd claim. They listen much more closely to individuals whose personal credibility they already know. Whatever the reason for this effect, astronomers would be wise to take advantage of personal acquaintances among government officials to fight light pollution.

Some local governments force dark-sky advocates to radical positions, as has happened in defense of Palomar Mountain Observatory near San Diego. There, John and Stephanie Mood write and speak from a radicalized stance, having learned that their local politicians respond to nothing less (Mood and Mood 1985).

Politics also affects other aspects of the struggle. Sky & Telescope magazine frequently plants ideas with its readers. A major article fighting light pollution was delayed more than a year because of political considerations: an amateur astronomer who wrote it joined S&T's rival, Astronomy magazine, and certain S&T editors were exceedingly reluctant to publish his by-line (Pike and Berry 1978). This phobia delayed the American fight against light pollution.

Major differences remain between the professional and amateur astronomers' advocacies. Professional astronomers, who do lots of spectroscopy, emphasize narrow-spectrum lighting and early-morning turnoffs. Amateurs, whose work is usually broad-band in the evening, campaign for hooding lights.

The Developing World

Much of the second and third worlds have yet to follow the path to development and pollution. Even now there are many places where light pollution is still minimal – amateur observers I have visited in Arusha, Tanzania, and Faaa, Tahiti, are blissfully unaffected. Their towns are small and concentrated, and use relatively low technology. They have no history of light pollution.

Elsewhere, different political systems impose different attitudes. I visited a very nice amateur club observatory in the capital city of a military dictatorship. Street lights glare almost all the way up to their doorstep. But they adamantly refuse to approach officials because "it is best if the government doesn't notice you at all." There, too, the solution is political rather than scientific, but necessitates terminating the dictatorship – obviously a bigger problem than light pollution. I am greatly distressed to notice that many countries, in their rush to build American-style industry and infrastructure, seem determined to repeat, as well, every mistake we have made in our own development. Those countries should notice the fabulous price that America now pays to restore its environment, mostly to undo our previous mistakes. Among these mistakes are many which contribute to air and light pollution. I fervently urge other countries to learn from our mistakes and deliberately avoid them, on their ways to development. "Those who do not learn from history are condemned to repeat it."

A Recipe to Change the Future

Though this history of light pollution offers some inspiration, it also teaches that we are still losing, and losing badly. Present tactics win only limited, local victories, while light pollution increases, even where restricted by ordinances. Therefore, this history suggests that we must take other approaches.

Eliminating waste lighting will help all astronomers, and also the consumers who pay for lighting, while conserving the resources that would otherwise be used to generate the waste. Sometime, there will be another energy crunch, or economic setback, or major reverse in public approval of utilities. To be ready when that happens, our coalition should design a new type of luminaire to satisfy our own criteria along with the public's. I think these include:
no light above 15° below horizontal.
smooth spread of light over target area on ground.
target area easily tailorable, perhaps by adjustable side shields.
2 or 3 narrow emission lines (perhaps from separate tubes, phosphorescence, or by other means) that the human eye will accept for decent color rendition, but at wavelengths not critical to spectroscopy, dim to color-film emulsions, and easy and cheap to filter out.
easy and cheap to manufacture.
rights easy to license to everyone who will make them.
economical to operate (easy installation and replacement, low power consumption, long life).

We should develop this luminaire. When we have it ready, we should promote it with all manufacturers, utilities, government authorities, and the public. When we offer a luminaire demonstrably superior for the public's purposes, it should win widespread acceptance, especially in an energy crunch. Since we also tailor it to suit our own purposes, that is how we can achieve our eventual victory.

All you pupils in this short-course want to select telescopes. Great! You have an idea that skywatching can be a nice hobby. In fact, it is sensational. And you think that a telescope will show some impressive sights. In fact, the views can be awesome. If you pay moderate attention you can learn enough to buy acceptable telescopes. But it's a much brighter idea to learn the ins and outs and get a telescope tailored to your wants.

Selecting a telescope can bewilder the beginning astronomer. There are so many types -

"reflectors," "refractors," and "compound catadioptric" systems like Maksutovs and Cassegrains. Manufacturers cunningly select specifications to make systems sound too good to be true. So how do you choose?

At First Glance

You're likely to look first in a book or magazine. Most of these blithely recommend getting the fattest telescope - that is, the greatest aperture, or width - you can afford. That advice indeed achieves a lot of very useful light-gathering power. Unfortunately, it also limits portability, and it is heavily biased toward Newtonian reflectors that are not optimal for some uses. Other sources proclaim the unexcelled view through refractors, although that's true mostly for planets and double stars. Through the 1950s, those were the most popular targets for amateurs, but no longer. Still other authorities tout the benefits of Schmidt-Cassegrains and Maksutovs, the compound catadioptric types, especially for astrophotography. This advice, too, should be restricted, mostly to those needing extreme portability.

What a Telescope Does

Think of a telescope simply as a tool to funnel light. There are just 2 basic things the funnel can do: It can spread light out, or it can concentrate it. To spread light out is to magnify the image. This enlarges the object in view, which is usually good, but it dilutes the brightness, which isn't. High-power images also have tiny fields of view, and this makes targets hard to find. The other alternative is to concentrate light, to shrink the object in view. This is usually not so good, but it also makes the image bright and contrasty, which is. Low-power images have wide fields of view that take in lots of stars. In fact, telescopes designed for this are nicknamed "rich-field" telescopes.

The steepness of the funneling is the focal ratio. Many optical instruments, especially cameras, express this as an f/number. This is simply the focal length (how far away light focuses) divided by the diameter of the opening where light enters. For example, if the diameter is 100 mm, an f/4 reaches focus at 400 mm, while an f/15 reaches focus at 1500 mm. For most refractors and Newtonian reflectors, the focal length determines the tube length. That, in turn, affects the height of the mounting and, therefore, its weight.

Telescopes have dozens of qualities to optimize. But no telescope is best at all the things telescopes can do. You can optimize some but inevitably at the cost of others. Principles of optics and physics extract a price for every gain. So telescope design is an art of tradeoffs. To get the most-desired qualities, others must be sacrificed - preferably ones you can live without. The "3 Laws of Telescope Design" are:

1. Every time you gain something, you lose something.

2. Every time you gain much, you lose more.

3. There's no such thing as a free lunch.

If these look familiar, it's because they seem to be the laws of everything else, too.

So the first step is to define what the telescope must do. The dominant question is: What kind of objects do you most want to view? Other important questions include: Where will you observe from? What about carrying the telescope by car, or by muscle? How perfect must the system be? And, of course, how expensive?

The objects you want to see should determine the focal ratio. A behavioral look at optics provides a few quick answers. It turns out that for solar system observing, long-focal-ratio refractors are superior. For the galaxies, nebulae, and clusters - deep sky observing - use the shortest and fattest system possible, and that usually means a stubby Newtonian when other factors are accounted for.

Compromises

And if you want to observe both within the solar system and beyond it, compromise. Get 2 telescopes: One each for the different types of viewing. If, however, it must be just one single telescope, there are choices to weigh. Only one configuration is both long and short - the Newtonian/Cassegrain - but only one small US producer has made them.

Or select a compromise focal ratio. Instead of the f/15 to 18 that shows the best detail on planets, or the f/4 to 6 that gives nebulae and galaxies the best contrast, try around f/10. Unfortunately, such systems usually deliver less-than-optimal images. To achieve the right magnifications for planets or for deep-sky objects, they require rather extreme eyepieces - either very short (less than 5 mm) or very long (more than 40 mm). Pushing optics to an extreme means a lot will have to be sacrificed to achieve even a little. Enormously long eyepieces are both expensive and heavy. Incredibly short ones are both difficult to construct and notoriously stingy on eye-relief, the ease with which you can see through them.

Poking Around the Neighborhood

Classic, long refractors are the "spyglass" type that leaps to most people's minds any time the word "telescope" comes up. Refractors team up lenses of at least 2 kinds of glass - commonly crown and flint - in a way that minimizes the chromatic aberration (spurious color) around bright images. This works best with focal ratios longer than f/11. New designs may work well at shorter ratios, but they will probably cost a lot. And their exotic, new types of glass may suffer problems of their own. So practical refractors are optically long. They deliver high magnification from conventional-length eyepieces, because magnifying power is simply the focal length of the objective divided by the focal length of the eyepiece.

Refractors are optimal for viewing the planets, Moon, and Sun. Their unobstructed light paths deliver the crispest and sharpest images. Planets appear quite small in the sky, as do details on the Moon and Sun, so you want to magnify them a lot. High magnification spreads out the image of an object, and that dilutes the light. But planets appear quite bright, so there's no problem. The classic long refractor need not be, therefore, too wide. The aperture gathers light, and the Sun, Moon, and planets offer plenty. This keeps the width, bulk, and cost of the telescope down.

Peering Far Beyond

Since the 1960s, observers have been flocking to deep-sky objects. This is due partly to the aperture explosion: Amateurs can now afford telescopes wide enough to gather enough light to make faint star clusters, nebulae, and galaxies impressive. Another stimulus was the incessant "Deep-Sky Wonders" column in Sky & Telescope magazine, written by Scotty Houston starting September 1946. Readers who initially passed over it eventually read a bit, then more and more until they were hooked. The star clusters, nebulae, and galaxies sought by amateurs have a lot in common: most appear much larger than planets, but vastly fainter. They are notoriously elusive, too. Some are so pale that it can take a long time to search them out.

The stubbier a telescope's focal ratio, the lower its magnifying power, so the better it concentrates the diffuse light of these objects. At first, it seems contradictory to use the lowest power on the farthest objects. But high magnification would produce a tangle of problems. The high-power field of view is tiny, and this makes it hard to locate and identify the right place in the sky. When you finally find it, only a small portion of the object may fit in at a time. And its light is so diluted, the image is washed out. You can scarcely tell anything is there at all. A short focal ratio delivers low power. The large field of view accommodates both the target and enough stars to facilitate identification. Also, it concentrates the diffuse light, enhancing contrast. This leads to an important supplementary principle to the laws of telescope design:

The bright ones are short, fat, and wide.

Novices find deep sky objects much more readily in short-ratio telescopes, and experienced amateur astronomers notice more detail through them.

Short-ratio telescopes are almost always Newtonians. That's mostly by elimination: It is difficult and expensive to build short refractors unless chromatic aberration grows objectionably. Compound telescopes gain most of their advantage by being compact. Compared to an already-compact reflector, they add little convenience, but they do cost a lot more. The remaining alternative is the Newtonian. There has been a gratifying flood of stubby Newtonians since the "Astroscan" appeared in 1976 and demonstrated that people would, indeed, buy a low-power telescope.

Limits

All this hints at limits to telescope capabilities that are only incidental to the optical pattern used. These limits are so remote from the beginning telescope purchaser that they are undreamt of. The truly limiting factors in designing a telescope for amateur skywatchers are not in the telescope itself! Instead, they result from phenomena beyond its ends.

Brightness

On the top end, the limiting factor is the surface-brightness of the object viewed. Surface brightness is its apparent brightness divided by its apparent area. Nature provides surface brightnesses only in 2 radically different families: "high" - in the Sun, Moon, and planets, and "low" - for nebulae, clusters, and galaxies. There is virtually nothing in between. Only fleetingly will a bright comet straddle that interval.

The gap in surface brightness results from a void in distance: Our star brilliantly illuminates only its local neighborhood, so nearby planets appear bright. Then there's a huge gap to stellar realms beyond. Between us and the next-nearest system (alpha Centauri) yawns an abyss of more than 4 light years. Since light's intensity diminishes with the square of the distance, light from beyond the solar system invariably appears radically fainter.

For example, compare 2 popular targets for amateur astronomers' telescopes. The planet Jupiter shines at about magnitude -2. Because its diameter is about 2/3 arcminute, its angular area is about 0.35 square arcminute. By contrast, the Dumbbell Nebula, M 27, is much larger and dimmer. At magnitude 8, it is 10,000 times fainter than Jupiter. M 27 spans 8 arcminutes by 5 arcminutes, or 40 square arcminutes. This is 115 times larger in area than Jupiter. The Dumbbell Nebula's surface brightness is, therefore, about 1,150,000 times less than Jupiter's. No wonder different optical systems are needed to show each at its best.

So, on the top end, telescopes are constrained by the surface brightnesses of their targets.

Pupils

On the bottom end, the limiting factor is not so much eyepieces as the human eye itself. The dark-adapted eye's pupil is rarely much over 6 mm wide. In young people the pupil can stretch to 7 mm, but the pupils of older folks don't exceed 5 mm. Also, smoking shrinks the pupil's ability to open widely. The telescope-and-eyepiece combination must be tailored to this. Any light that arrives wider than the pupil cannot enter the eye, and is thus sheer waste. So the telescope's exit-pupil must not exceed about 6 mm. The exit-pupil is simply the objective's diameter divided by the magnification. For any given telescope, the exit pupil enlarges as the power shrinks - that is, as the eyepiece lengthens. For a nice long eyepiece with low power and wide-field, contrasty views of deep-sky objects, the exit pupil must be large. Up to about 6 mm that's fine; beyond, there is no gain. The longest common eyepieces, used with conventional telescopes, deliver exit pupils around this size.

Therefore, in determining what telescope to make or buy, the paramount considerations are not in the telescope itself. The limiting factors are the surface-brightnesses of the objects you observe, and the entrance pupil of your dark-adapted eye. Tailor a telescope - a light-funnel - to fit between those objects and the eye, so that the second will receive the optimal view of the first.